Quantitative aspects of the binding of human IgG erythrocyte autoantibodies to red cells are being studied by a highly sensitive version of the antiglobulin consumption test. This method utilizes quantitative complement fixation reactions to measure consumption of anti-IgG antibody by absorbing red cells. Under study are (a) cases of hemolytic anemia with negative direct Coombs test, (b) quantitative changes in cell-bound autoantibody associated with corticosteroid therapy, and (c) auto-antigen density on human red cells. Also under study are newly recognized kindred with hereditary deficiencies of C6 or C2, and the functional performance of such complement-deficient human sera in bactericidal, chemotactic and cytolytic assays (PNH red cells), and the relation of the complement deficiency to hemostatic functions.